TypeScript 4 upgrade follow-ups

[ecraig12345]

#17932 upgrading to TS 4.1 has been merged, but there are still some follow-ups needed.

• Small unaddressed PR comments - PR Fix minor unaddressed TS 4 upgrade PR comments #18027
• Add a test to ensure basic compatibility with TS 3.9. - Add a test to ensure compatibility with TS 3.9 #18440
• Update eslint-related deps to compatible versions - PR Update eslint deps for compatibility with new TS version #18024
• Update prettier to compatible version (needed for export type) - tracked by Upgrade prettier #18008, PR Update prettier to latest version and format all files #18041
• Update ts-jest to compatible version - Update jest and ts-jest to 27 #18442

Possible future follow-ups

• Full type downleveling/typesVersions - Add type downleveling (typesVersions) #18441

Questions

• What is our consumer TS min bar for converged?
• What is our consumer TS min bar for northstar? 4.1.5

[layershifter]

• What is our consumer TS min bar for northstar?

"typescript": "4.1.5", so we don't need to integrate downleveling into Gulp scripts currently.

[ecraig12345]

Closing in favor of individual tracking issues (linked in updated issue description).